\psset{unit=0.8,algebraic}
\begin{pspicture*}(-7,-1)(7,4)
\psaxes[labelFontSize=\scriptstyle,ticksize=-3pt 0,labelsep=2pt]{<->}(0,0)(-3,-1)(3,4)
\psplot[linecolor=blue]{-3}{3}{2^x}
\psplot[linecolor=blue]{-3}{3}{0.5^x}
\psplot[linecolor=blue]{-3}{3}{2.7183^x}
\footnotesize
\rput[l](1.5,2){$f(x)=2^x$}
\rput[r](-1.5,2){$f(x)=0.5^x$}
\rput[l](0,3.5){$f(x)=e^x$}
\end{pspicture*}